This invention relates to a polyurethane (PU) prepolymer containing isocyanate (NCO) groups and having a low content of monomeric polyisocyanate, to its production and to its use.
Polyurethane prepolymers containing NCO groups, preferably terminal NCO groups, have been known for some time. They may readily be chain-extended or crosslinked with suitable hardeners—generally polyhydric alcohols—to form high molecular weight materials. Polyurethane prepolymers have acquired significance in many fields of application, for example in the production of adhesives, coatings, casting resins and molded articles.
In order to obtain NCO-terminated PU prepolymers, it is common practice to react polyhydric alcohols with an excess of monomeric polyisocyanates—generally at least predominantly diisocyanates. The molecular weight can be at least approximately controlled through the ratio of OH groups to isocyanate groups. Thus, a ratio of OH groups to isocyanate groups of or approaching 1:1 generally leads to high molecular weights. However, with a ratio of about 2:1, for example, where diisocyanates are used, a statistical average of one diisocyanate molecule is attached to each OH group so that, ideally, no oligomerization or chain extension occurs during the reaction.
In practice, however, such chain extending reactions cannot be suppressed with the result that, at the end of the reaction, a certain amount of the component used in excess is left over, irrespective of the reaction time.
If, for example, a diisocyanate is used as the excess component in the reaction of diisocyanates with polyhydric alcohols, generally at least predominantly diols, and if the isocyanate groups of the diisocyanate have substantially the same reactivity, around 25% of the monomeric diisocyanate used remains as monomer in the prepolymer according to the Schulz-Flory statistic, for example for an NCO/OH reaction ratio of 2:1.
The presence of monomeric polyisocyanate is problematical, for example, when readily volatile diisocyanates have been used as the monomeric polyisocyanate. Adhesives/sealants and, in particular, PU-based hotmelt adhesives are applied at elevated temperature. Thus, the application temperatures of hotmelt adhesives are in the range from 100° C. to 200° C. while those of laminating adhesives are in the range from room temperature to 150° C. Even at room temperature, volatile diisocyanates, such as IPDI or TDI, have a significant vapor pressure. This significant vapor pressure is serious above all in the case of spray application because, in this case, significant quantities of isocyanate vapors can occur over the application unit. Isocyanate vapors are toxic in view of their irritating and sensitizing effect. The use of products with a high content of readily volatile diisocyanates involves elaborate measures on the part of the user to protect the people responsible for applying the product, more particularly elaborate measures for keeping the surrounding air fit to inhale, as legally stipulated by the maximum permitted concentration of working materials as gas, vapor or particulates in the air at the workplace (annually updated “MAK-Wert-Liste der Technischen Regel TRGS 900 des Bundesministeriums für Arbeit und Soziales”).
Since protective and cleaning measures generally involve considerable financial investment or costs, there is a need on the part of the user for products which—depending on the isocyanate used—have a low content of readily volatile diisocyanates.
“Readily volatile” substances in the context of the present specification are substances which have a vapor pressure of more than about 0.0007 mm Hg at 30° C. or a boiling point of less than about 190° C. (70 mPa).
If low-volatility diisocyanates, more particularly the widely used bicyclic diisocyanates, for example diphenylmethane diisocyanates, are used instead of the high-volatility diisocyanates, the PU prepolymers or adhesives based thereon generally obtained have viscosities that are normally outside the range relevant to simple methods of application. This also or additionally happens where it is intended to reduce the monomer content by reducing the NCO:OH ratio. In these cases, the viscosity of the polyurethane prepolymers can be reduced by addition of suitable solvents, but this goes against the general demand for no solvents. Another way of reducing viscosity without using solvents is to add an excess of monomeric polyisocyanates as so-called reactive diluents. These reactive diluents are incorporated in the coating or bond in the course of a subsequent hardening process (after addition of a hardener or by hardening under the effect of moisture).
Although the viscosity of the polyurethane prepolymers can actually be reduced in this way, the generally incomplete reaction of the reactive diluent and or, generally, the presence of monomeric unreacted starting polyisocyanate often leads to the presence in the bond or coating of free monomeric polyisocyanates which are capable of “migrating”, for example, within the coating or bond or, in some cases, even into the coated or bonded materials. Such migrating constituents are commonly known among experts as “migrates”. By contact with moisture, the isocyanate groups of the migrates are continuously reacted to amino groups. The content of the amines, particularly primary aromatic amines, thus formed must be below the detection limit—based on aniline hydrochloride—of 0.2 micrograms aniline hydrochloride/100 ml sample (Bundesinstitut für gesundheitlichen Verbraucherschutz und Veterinärmedizin, BGVV, nach amtlicher Sammlung von Untersuchungsverfahren nach § 35 LMBG—Untersuchung von Lebensmitteln/Bestimmung von primären aromatischen Aminen in wässrigen Prüflebensmitteln).
Migrates are undesirable in the packaging industry and particularly in the packaging of foods. On the one hand, the passage of the migrates through the packaging material can lead to contamination of the packaged product; on the other hand, long waiting times are necessary—depending on the quantity of migratable free monomeric polyisocyanate—before the packaging material is “migrate-free” and can be used.
Another unwanted effect which can be caused by the migration of monomeric polyisocyanates is the so-called antisealing effect in the production of bags or carrier bags from laminated plastic film. The laminated plastic films often contain a lubricant based on fatty acid amides. By reaction of migrated monomeric polyisocyanate with the fatty acid amide and/or moisture, urea compounds with a melting point above the sealing temperature of the plastic films are formed on the surface of the film. This leads to the formation between the films to be sealed of a “foreign” antisealing layer which counteracts the formation of a homogeneous sealing seam.
However, problems are caused not only by the use, but also the by the marketing of reactive adhesives containing monomeric polyisocyanate. In connection with consumer protection, safety and occupational hygiene, the obligation to identify isocyanate-containing products was made even more strict as from 1 Jun. 2000. Thus, substances and preparations containing, for example, more than 0.1% free MDI or TDI come under the law on hazardous materials and have to be identified accordingly. The obligation to do so involves special measures for packaging and transportation.
Methods of obtaining PU prepolymers with a low percentage content of monomeric polyisocyanate, generally monomeric starting diisocyanate, and optionally avoiding the above-described disadvantages or improving the physiological properties of PU-based adhesives are known.
DE-PS 953 012 describes a process for the production of soluble, relatively high molecular weight polyisocyanates capable of further reaction. In this process, polyhydric, low molecular weight alcohols are reacted with such quantities of diisocyanates that, for every hydroxyl group, there are more than one and less than two isocyanate groups. The particular value of these new polyisocyanates is that, by virtue of their relatively high molecular weight, they show hardly any vapor pressure and are therefore physiologically harmless. Suitable polyalcohols are inter alia ethylene glycol, hexanediol, diethylene glycol, methyl hexanol. The diisocyanates mentioned include 4,4′-diphenylmethane diisocyanate, hexamethylene diisocyanate, toluene diisocyanate.
DE 37 39 261 A1 relates to a process for the production of polyisocyanates containing urethane groups by reacting aromatic diisocyanates with polyhydric alcohols having a molecular weight of 62 to 250 and subsequently removing the unreacted excess starting diisocyanate by special distillation. 2,4-Diisocyanatotoluene or technical mixtures thereof with 2,6-diisocyanatotoluene are preferably used as the diisocyanates. The end products of the process are distinguished by a particularly low content of free starting diisocyanate of under 0.3% by weight.
DE 41 40 660 A1 describes polyisocyanates containing ether and urethane groups which are produced by reacting polyhydroxy polyethers having a molecular weight of 350 to 500 with excess quantities of toluene diisocyanate and subsequently removing any unreacted excess of this starting diisocyanate by distillation to a residual content of less than 0.1% by weight.
DE 42 32 015 A1 describes solventless two-component polyurethane adhesive systems characterized by high early strength and low migration values based on hydroxyl polyesters and prepolymers containing isocyanate groups. The prepolymer containing isocyanate groups is prepared from polyether polyols with an average molecular weight of 400 to 1,500 and 2,4- or 2,6-toluene diisocyanates or mixtures thereof. The use of a large excess of toluene diisocyanate, i.e. an NCO to OH group ratio of greater than two, leads to prepolymers with a monomer content which, although high, can be reduced to below 0.15% by weight by distilling off or extracting the excess monomer, optionally using an inert entraining agent. Such prepolymers are distinguished by particularly low viscosities and an isocyanate content of 4 to 11% by weight.
WO 98/29466 describes a low-monomer PU prepolymer which is prepared in two reaction steps. In a first reaction step, a partly sluggishly reacting (nonsymmetrical) diisocyanate, preferably TDI, is reacted with polyhydric alcohols in an OH:NCO ratio of 4 to 0.55:1. After virtually all the fast NCO groups have reacted off with some of the OH groups present, a more reactive diisocyanate (symmetrical diisocyanate), preferably MDI, is added in less than the equivalent quantity, based on the still free OH groups, in a second reaction step.
EP 0 019 120 A1 describes a process for the production of elastic, weather-resistant flat materials using a moisture-curing prepolymer obtained by reacting equimolar quantities of a polyol, reducing the monomeric TDI content to below 1% by thin-layer distillation and then reacting the reaction product with diphenylmethane diisocyanate and a polyol. The prepolymer contains 5 to 15% by weight of free NCO groups.
Despite the prior art cited above, there was still a need for improved PU compositions with a low monomeric polyisocyanate content which would be migrate-free and, in particular, would have an optimal viscosity for the particular application envisaged, could be processed sufficiently quickly and safely and would be particularly suitable for use in the packaging industry.